In recent years the automobile manufacturing industry has been offering an optional window accessory with which frost and/or condensation can be removed using an electrically conductive grid that is permanently installed in the window. For rapid defrosting, it must be possible to supply a large amount of power, such as 12 volts, from a low voltage power source to the circuits. Furthermore, the wires of the conductive grid must be thin enough that they will not block the visibility out the rear window.
The resistance of the conductive pattern needs to be on the order of 2 to 30 milliohms per square, and this requirement is easily met by a noble metal conductor, and particularly silver. Silver is the most widely used conductor substance in this application at the present time.
Almost all of the substances used up to now in the manufacture of grids for removing window condensation contain a thick film silver conductor, which is manufactured from a paste composed of fine silver particles and a glass frit dispersed in an organic medium. In a typical application, a paste containing 70 wt % silver powder, 5 wt % glass frit, and 25 wt % organic medium is screen printed through a 180 standard mesh screen onto a rear window made of flat, unmolded glass. The printed composition is dried for at least two minutes at approximately 150.degree. C., and the entire element is then fired for two to five minutes in air at 650.degree. C. After this firing, the softened glass is compressed into a mold and molded, then rapidly cooled and annealed. During the firing cycle, the organic medium is removed by evaporation and pyrolysis. The glass and the silver are sintered, with the glass acting as a binder for the silver particles, and this forms a continuous conductive path.
As shown in FIG. 1, the automobile window defogger requires a wide surface area (about 1 cm wide by 50 cm long) portion called a bus bar 3 that is used to bond electrode terminals to the grid 2 positioned over the rear window 1 of the automobile. A black glass paste called enamel is printed between the glass sheet and the silver so that the bus bar portion 3 cannot be seen from outside the automobile. In this case, since the enamel and the silver paste are fired at the same time, the glass in the enamel rises to the surface of the silver, where it creates problems of poor terminal soldering or poor adhesive strength. Methods that have been adopted in the past to deal with these problems including printing twice in order to make part of the silver thick film thicker, and printing only the terminal bonding portion with enamel, but the drawbacks to these methods were that they increased the number of steps in the process or increased the amount of silver used.
The present invention offers a thick film conductive paste that has high adhesive strength and good terminal solderability in simultaneous firing with the enamel, even if the thick film of the silver paste is made thicker.